Does higher growth rate in Atlantic cod (Gadus morhua) at low salinity result from lower standard metabolic rate or increased protein digestibility?

This study examines whether increased growth rate in Atlantic cod (Gadus morhua) at low salinity (14%) can be accounted for by a reduction in standard metabolic rate (SMR) or by an increase in food assimilation (protein digestibility). Cod tolerate low salinity and exhibit higher growth rates at low salinity than in seawater. Differences between salinities are not associated with differences in food ingestion rates. SMR measured in cod exposed to low salinity and seawater (28%) did not decrease or increase with changes in salinity. Similar results were obtained for cod with and without salinity acclimation. Weight loss in cod acclimated to salinity was similar at low salinity as in seawater. No difference in apparent protein digestibility was observed between cod acclimated to low salinity (14%) and seawater. Estimates of protein digestibility were similar to those reported for cod in earlier studies. Higher growth rates and higher food conversion ratios at low salinity in cod must be attributable to uncontrolled or unmeasured factors such as spontaneous activity and swimming performance rather than to changes in SMR or in protein digestibility.